Louver windows and doors have long proved an easy way to obtain illicit entry into a building with little effort and noise. Thus, in the absence of a restraining element, a single pane or panel might be pushed up with its end brackets until it could be lifted out of place, and then by an arm inserted through the opening thus formed, other panels easily removed and/or the door latch thus manipulated from the outside. In my U.S. Pat. No. 3,903,649, I disclosed a security clip or liner which could be mounted, one on each end of a louver panel, and snugly lodged (permanently) within the corresponding pair of end brackets of the assembly so as to resist external efforts to dislodge it. The present invention provides a modification of such liners so that they can now be mounted interchangeably on either end of the panel (i.e. do not have to be supplied as right-end and left-end forms) and in addition provides positive locking means for transversely directed anchorage of the liner with the bracket and both of them with a retainer element. When thus locked, the panel is resistant to upward displacement even by repeated hammer blows directed against its lower outer edge.